The present invention relates to a method of cutting a semiconductor wafer by means of plasma etching. The present invention also relates to a protective sheet used in the method of cutting the semiconductor wafer.
A semiconductor device mounted on a board of electronic equipment is conventionally manufactured in such a manner that pins of a lead frame and metallic bumps are connected to semiconductor elements, on which a circuit pattern is formed in the state of a wafer, and the semiconductor elements are subjected to a packaging process so that they can be sealed with resin. Since the size of electronic equipment has been recently reduced, the size of the semiconductor device has been also decreased. Especially, investigations are actively made into the reduction of the thickness of a semiconductor element.
The mechanical strength of the semiconductor element, the thickness of which is reduced, is so low that the semiconductor element is liable to break in the process of cutting conducted in the dicing step in which the semiconductor element in the state of a wafer is cut into individual pieces, and the yield of machining is inevitably lowered. Concerning the method of cutting the semiconductor element, the thickness of which is reduced, instead of the mechanical cutting method, a plasma dicing method is proposed in which the semiconductor wafer is cut when cutting grooves are formed by the etching action of plasma. Concerning this method, for example, refer to Japanese Publication JP-A-2002-93752.
However, in the process of plasma dicing of the prior art described above, due to the want of uniformity of etching action conducted by plasma, the following problems are caused and have not been solved yet. In the process of plasma etching, masking is conducted on a semiconductor wafer in which regions except for the cutting lines are covered with a resist layer. After the completion of masking, the semiconductor wafer is accommodated in a processing chamber in a plasma processing device, and only regions of the cutting lines are exposed to plasma in the processing chamber so as to remove these portions by means of etching.
In this connection, an etching rate showing the degree of etching conducted by plasma is not necessarily uniform. Therefore, the etching rate distribution fluctuates in the processing chamber. Accordingly, in the process of plasma dicing conducted in the processing chamber, silicon in the portions of the cutting lines, which are located in a range of a high etching rate, is more quickly removed than silicon in the other portions. Therefore, cutting is more quickly completed in these portions.
The cutting lines in these portions of the high etching rate, are successively exposed to plasma until silicon in the portions of the cutting lines located in regions of the low etching rate is removed. Accordingly, when silicon is completely removed from the regions of the high etching rate, the protective sheet on the lower face side of the semiconductor wafer is directly exposed to plasma.
When the plasma processing continues in the above state, heat generated by plasma directly acts on the protective sheet. As a result, there is a possibility that the protective sheet is superheated, burned and deformed. According to the conventional plasma dicing method, it is impossible to effectively prevent the protective sheet from being damaged by heat caused by the want of uniformity of the etching action of plasma.